Since rigid polyurethane foams and isocyanurate-modified rigid polyurethane foams are excellent in heat insulation and self-adhesiveness, they are widely used as heat-insulating materials for electric refrigerator, building materials, and the like. The rigid polyurethane foams and isocyanurate-modified rigid polyurethane foams to be used for these applications are generally obtained by a method of mixing, with a polyisocyanate, a raw material-blended composition having mixed therewith a polyol component, a blowing agent, a catalyst, a foam stabilizer and the other additives, and causing a blowing reaction. In many cases, a raw material-blended composition for producing the rigid polyurethane foam and the isocyanurate-modified rigid polyurethane foam is stored for from several weeks to three months until the actual use after blending. Namely, since the raw material-blended composition is used after the passage of several weeks to three months from its blending, the storage stability becomes the issue.
Dichloromonofluoroethane (HCFC-141b) currently used as a blowing agent for the rigid polyurethane foams or isocyanurate-modified rigid polyurethane foams has a problem of ozone-layer destruction. Therefore, as a next-generation blowing agent instead thereof, hydrofluorocarbon (hereinafter sometimes referred to as HFC) without destroying an ozone layer is lined up as a substitute. As HFC, there are tetrafluoroethane (HFC134a), 1,1,1,3,3-pentafluoropropane (HFC245fa), 1, 1,1,3,3-pentafluorobutane (HFC365mfc), 1,1,1,2,3,3,3-heptafluoropropane (HFC227ea), and the like.
Moreover, a low-boiling hydrocarbon (hereinafter sometimes referred to as HC) without destroying an ozone layer is also regarded as a strong substitute. As examples of such a hydrocarbon (HC), hydrocarbons having a boiling point of −30 to 70° C. are used. As specific examples thereof, there are known propane, butane, pentane, cyclopentane, hexane, and mixtures thereof.
However, in the case of using a conventional catalyst, there is a problem that a raw material-blended composition containing such a blowing agent is poor in storage stability.
Moreover, in order to obtain a low-density foam, water is used as a blowing agent other than HC and HFC. In the case of using water, carbon dioxide formed in the reaction of water with a polyisocyanate component is utilized as a blowing component. Furthermore, it is also possible to use HC or HFC in combination with water.
However, in the case that water which generates a blowing component is contained in the raw material-blended composition, there is a problem of particularly poor storage stability of the raw material.
The reaction of forming a rigid polyurethane foam mainly comprises a urethane group-forming reaction by the reaction of a polyol with a polyisocyanate (resinification reaction) and a urea group-forming reaction by the reaction of a polyisocyanate with water (blowing reaction). Moreover, the reaction of forming an isocyanurate-modified rigid polyurethane foam comprises an isocyanurate ring-forming reaction by trimerization of a polyisocyanate (isocyanurate reaction) in addition to the above two kinds of reactions. The catalyst to be used in these reactions exerts large influence on not only the reaction rate but also the thermal conductivity of the foam, the curing rate of the foam surface, adhesive strength, moldability, dimensional stability, physical properties, and the like. Industrially, a viewpoint of storage stability is particularly important.
In this connection, for the reason of improving flame retardancy and physical properties, an aromatic polyester polyol obtained by esterification of an aromatic dicarboxylic acid is frequently used as a polyol component for rigid polyurethane foam and/or isocyanurate-modified rigid polyurethane foam products.
Conventionally, as a catalyst for producing rigid polyurethane foams, a compound of particularly promoting the resinification reaction and/or blowing reaction is used. As such a catalyst, an organometallic compound or a tertiary amine compound has been hitherto used. For example, as the tertiary amine compound as the catalyst for producing polyurethane foams to be used industrially, there are known compounds such as triethylenediamine, N,N,N′,N′-tetramethyl-1,6-hexanediamine, N,N-dimethylcyclohexylamine, bis(2-dimethylaminoethyl)ether, and N,N,N′,N″,N″-pentamethyldiethylenetriamine.
Moreover, as a catalyst for producing isocyanurate-modified rigid polyurethane foams, there are known organometallic catalysts such as alkali metal salts of carboxylic acids, metal alcoholates, metal phenolates, and metal hydroxides, tertiary amines, tertiary phosphines, onium salt compounds of phosphorus, quaternary ammonium salts, and the like as catalysts of particularly promoting the isocyanurate-forming reaction. Of these, alkali metal salts such as potassium acetate and potassium 2-ethylhexanoate, quaternary ammonium salt-based catalysts such as quaternary hydroxyalkyltrimethylammonium 2-ethylhexanoate salt, S-triazine compounds such as 1,3,5-tris(N,N-dimethylaminopropyl)hexahydro-S-triazine, and specific tertiary amines such as 2,4,6-tris(dimethylaminomethyl)phenol are widely used because of high isocyanurate-forming activity. Furthermore, as the quaternary ammonium salts, tetraalkylammonium salts such as tetraalkylammonium organic acid salts are known (see, for example, Patent Document 1 for tetraalkylammonium organic acid salts).
However, in the case of using these catalysts for producing rigid polyurethane foams and isocyanurate-modified rigid polyurethane foams, there is a problem that a polyester polyol in a raw material-blended composition is tend to be hydrolyzed in the present of water or a blowing agent containing water and an amine-based catalyst, and thus the storage stability of the raw material-blended composition decreases to result in impossible preparation of normally blown products. For solving the problem, an improvement of the polyol component and influences of flame retardant and catalyst have been investigated but a sufficient solution has not yet been proposed (see, for example, Non-Patent Document 1 for storage stability of raw material-blended compositions).
Patent Document 1: Japanese Patent No. 3012897
Non-Patent Document 1: MCADAMS et al., “Stabilization of rigid Systems Containing Aromatic Polyester Polyol and Water”, Polyurethane Conference 2002, Conference Proceedings, Page 3 to 8.